I was wondering I got two heat sinks and a copper plate what the hell is the copper plate for ? When I bought the raspberry PI 2 it came with green and black and copper. Green was the big heat sink and the black small and copper was a plate
There is exactly a picture showing how to use the coper plate from the link you given. It seems it's placed under the CPU at the back of the board, and be careful there is a plastic plate between the pins and the copper plate.
According to Raspberry Pi Foundation, you don't need a heatsink, if you use it normally, that is not overclocking. So I think it will be fine if you don't use it.